Aetherometer

"Tarralein statues mandate that any shop selling items of a thaumaturgical or alchymical nature be equipped with a minimum of one functional aetherometer, madam. Three guardsmen all agree that you did not possess such an item at the time of their inspection. I am afraid you have no choice by to pay the fine of five crowns, surrender goods equal in value to this amount, or be sent to jail."
-Magistrate Luchen, notorious racketeer, after sending his guards to break aetherometers

Full Item Description
Although the specifics vary from maker to maker, the rough details of an aetherometer are fairly simple; a mounting of some sort, typically a fancifully decorated tripod of brass with small runes worked into the metal, holds aloft a sphere of polished cobalt glass or other deep blue crystal, around which are wound wires of precious metals, binding it to the mounting and completing the thaumaturgic circuit. Unlike many devices of this sort, it has no thaumic battery to power it, drawing instead directly on the surrounding energy fields.
When active, the cobalt sphere generally glows, the brightness indicating the overall strength of the surrounding thaumaturgic fields.

"We can't rightly afford Aetheric Goggles for all our engineers, but these things are a sight cheaper and warn us when something's gone wrong. Then we can call in some of the lads what has the goggles to see what's wrong."
-Factory Manager Dubrille

History
Aetherometers actually predate Aetheric Goggles by a few centuries; prior to the rise of thaumatechnology, however, they were created and owned almost exclusively by mages who needed to know about the ambient energy in an area. Since the rise of steam technology, however, they have become a much more common sight, and in many cities are required by law in any shops which sell the products of thaumatechnology. Factory owners, in particular, have come to appreciate them, as they're a great deal cheaper over extended time periods than aetheric goggles, as they need no power source to function.

"It was a sight, I tell you. We walked into this alchymical shop to see if we could sell of some of the weird plants we found in the ruins we'd been exploring, when all of a sudden the store's aetherometer just explodes. Bits of glass in everything, a right mess.
Turns out the rusty old key our scout and locksmith found and decided to keep for luck was a relic of one of the Lost Gods. Apparently the bloke's an Immortal now, some minor god of locks or something."
-Glithri Quillhand, adventurer

Magic/Cursed Properties
Aetherometers are measures of the surrounding levels of energy; they absorb a tiny fraction of the ambient energy, converting it into light within the cobalt sphere atop the device. More complex models can display different colors or varying levels of light in different quadrants to measure specific fields, but the common model lumps all the energy into a single composite glow.
Most of the time, the spheres glow no brighter than a well-made candle. However, in shops or active factories, they can flare as brightly as a gas lamp; secondhand accounts also relate that in a 'dead' area, the blue glass turns black, and that the presence of a thaumavore such as a Whisp can cause the glass to frost over. In addition, particularly high concentrations of energy - such as the midst of a patch of thaumaturgic pollution - will cause the sphere to glow nearly white and often emit a low humming sound as it strains to contain the energy it is converting.

Inspired by comments by Ouroborus and Chaosmark on the Aetheric Goggles.